1. Field of the Invention
The present invention relates to a semiconductor integrated circuit comprising a circuit for testing and evaluating the AC timing of an external input/output terminal of a semiconductor integrated circuit.
2. Description of the Prior Art
In recent years, as the progress of semiconductor technologies has enabled semiconductor integrated circuits to operate at higher speeds, external input/output signals have been supplied at increasingly higher speeds to and from the external input/output terminals of the semiconductor integrated circuits. In such semiconductor integrated circuits operating at increasingly high speeds, an AC timing test on external input/output signals has become increasingly important as a mass-production shipment test.
For example, in the case of testing an AC timing using an input signal from the outside of a semiconductor integrated circuit as a reference, the AC timing can be tested by setting a test signal outputted from an LSI tester to a specified timing relative to the timing to which the reference input signal has been set.
By contrast, in the case of testing an AC timing using an output signal from a semiconductor integrated circuit as a reference, the AC timing can be tested by retrieving the transition point of the reference output signal by means of an LSI tester and setting the timing for determination by the LSI tester relative to the transition point such that it satisfies specifications.
There is also a semiconductor integrated circuit comprising an AC timing test circuit having, e.g., a timing test circuit and a self-diagnostic circuit, which uses an output signal to one external output terminal thereof as a reference to test the AC timing of an output signal to another external output terminal (see, e.g., Japanese Laid-Open Patent Publication No. 2001-221834).
In an AC timing test as described above, it is necessary to use an LSI tester capable of operating at a speed higher than the operating speed of an external input/output terminal. Accordingly, as the operating speed of a semiconductor integrated circuit has become higher, an LSI tester which satisfies increasingly higher-speed specifications has been in growing demand. However, because an LSI tester which satisfies higher-speed specifications is generally higher in cost, the use of a higher-cost LSI tester leads to an increase in testing cost. For this reason, in an LSI test during mass production, it is requested to perform an AC timing test on an external input/output terminal by using an LSI tester lower in operating speed than the external input/output terminal.
To satisfy the request, the above-mentioned semiconductor integrated circuit comprising the AC timing test circuit allows an LSI test using an LSI tester lower in operating speed than the external input/output terminal. However, the semiconductor integrated circuit with the AC timing test circuit has been proposed to perform the test on an output signal and it cannot perform the test on an input signal.